Method for preparing acrylonitrile synthetic fiber



p 1964 YosI-IIsATo FUJISAKI ETAL 3,147,322

METHOD FOR PREPARING ACRYLONITRILE SYNTHETIC FIBER FiledJan. 25, 1963 ACRVLON/TR/LE POLVMERAT LEAsT 85% BY WEIGHT ACRYLONIT- HAVING AVERAGE MOLECULAR RILE, BALANCE OF ETHYLENICALLY WEIGHT BETWEEN 60,000 a UNSATURATED HYDRDCARBON Ioopoo MONOMER DISSOLV/NG POLYMER N/7'R/C AC/D $0LV/VT-6680% BY WEIGHT /N 50 VENT TEMF. 30 To 5 c NITRIC ACID a LESS THAN 0.00o5% N ITROUS ACID SP/NN/NG $OLU770N WITH A VISCOSITY BETWEEN 700 a 2000 POISES AT o c EX THUS/ON FIRST --Ac1uEous SOLUTION coNTAINING BETWEEN BATH 2a a. se% BY WEIGHT NITRIC AcIo TEMP. -ao T0 5c COAGULA TED F/LAMENTS HAVING A SHRINKAGE BETWEEN I5 6 407 SECOND COAGULAT/ON AQUE0U5 SOLUTION CONTAINING BETWEEN BATH l5 s 25% BY WEIGHT NITRIC ACID WASHING R HEAT STRETCH/N6 FILAMENTS 7 To 1| -HOT WATER. 60 TO [00C TIM ES IN LENGTH INVENTORS YOSH/SA TO FUJ/SAK/ CH 020 NAKA WIMA HIDE H/KO KOBA VA SH/ United States Patent 3,147,322 METHOD FOR PREHARING ACRYLQNITRlLE SYNTHE'HC FIBER Yoshisato Fujisaki, Numazu-shi, and Chozo Nakayama and Hidehiko Kobayashi, Fuji-shi, Japan, assignors to Asahi Kasei Kogyo Kabushiki Kaisha, Kita-kn, Osaka, Japan, a corporation of Japan Filed Jan. 23, 1963, Ser. No. 253,269 1 Claim. (Cl. 264-482) This invention relates to the preparation of synthetic fiber from polymers and copolymers of acrylonitrile by the use of a nitric acid spinning method in which soft acrylic synthetic fibers are obtained, which are especially comfortable when directly in contact with the skin. This is obtained according to the invention by imposing a specific limit on such conditions as the molecular weight of the raw material polymer, the concentration thereof in a spinning solution or the viscosity of a polymer solution to be spun, as well as the solvents, spinning conditions, and drying conditions.

More particularly, the present invention concerns a method for producing acrylic synthetic fiber which comprises dissolving acrylonitrile polymer having an average molecular weight of 60,000-100,000, in nitric acid of more than 66% concentration which contains less than 0.0005 by weight of nitrous acid, at 5 C. or below, to

obtain a spinning solution having a viscosity of at least 700 poises at 0 C. and keeping the spinning solution at a temperature of 5 C. or below, and then extruding the spinning solution into an aqueous coagulation bath containing 28-36% by weight of nitric acid, kept at 5 C. or below, thereby to effect coagulation of the spinning solution while providing at least 15% shrinkage to the fibrous coagulation. The coagulated material is then fed into a 15-25 dilute aqueous nitric acid solution until coagulation is completed. The completely coagulated materials are washed with water to remove nitric acid from the fibrous material, and are then subjected to hot stretching in hot water of 100 C. or below to stretch the materials 7-11 times. The materials are dried at a temperature of 80 C. or higher. Acrylic synthetic fiber has been extensively developed and various products have been obtained by different processes. However, it is well known that they are different in feel, drape, and other physical properties in spite of their similarities in chemical compositions. In other words, the diiferences in manufacturing processes may largely change some of the properties of the textiles. An extremely abstract expression of good feel which is the great characteristic of acrylic fiber prepared by the method of this invention can not be numerically shown but it is true that the fiber prepared by this invention has a good feel to the touch. Since there is a considerable difference in touch and feel between the products of this invention and of other methods, it can be said that this invention is a very useful one from the commercial point of view.

In order to obtain the benefits of the present invention to produce acrylic fiber having good feel, especially against the skin, it is necessary to adopt a production process regulated by more strictly limited conditions which have never been taught in conventional methods. It should be noted that all the conditions such as molecular weight of the polymer, concentration of the solvent, temperature of spinning solution and coagulation bath, and other spinning conditions, and drying conditions for wet fibers after hot stretching are properly correlated with each other.

There is shown in the attached drawing a diagrammatic representation of the process according to the invention in the form of a flow sheet.

3,l47,322 Fatentecl Sept. 1, 1964 First of all, it is necessary to use as a solvent nitric acid containing at least 66% HNO and less than 0.0005% nitrous compounds, e.g. N0 HNO and salts thereof, N 0 and N 0 The reason that the said concentration of nitric acid is necessary is mainly based upon the fact that the dissolved state of acrylonitrile polymer in the spinning solution is greatly affected by the concentration of nitric acid employed, and further, the said solution state greatly affects the physical properties of the resulting fibers. When nitric acid of less than 65% HNO is used, satisfactory benefits of this invention can not be obtained.

Results of examinations regarding the effects of nitric acid concentration on the solution state, carried out by using the intrinsic viscosity of a dilute solution of the said polymer as a measure of the solution state, are as follows: polyacrylonitrile which has an intrinsic viscosity of 375 cc./ g. in dimethylformamide solution is dissolved in 55% nitric acid to produce an intrinsic viscosity of 240 cc./g.; in 60% nitric acid, 336 cc./g.; in 65% nitric acid, 401 cc./g.; in 70% nitric acid, 455 cc./g.; and in nitric acid, 478 cc./g., respectively. The intrinsic viscosity thus increases rapidly with increasing nitric acid concentration, and the increasing rate gradually lowers when the concentraton becomes higher than about 70%. In other words, in the case of lower nitric acid concentration, even a small difierence in nitric acid concentration would greatly affect the physical properties of the polymer solution.

Generally, it is desirable that solvents for preparing spinning solutions for the production of fibers be those which favorably dissolve starting material polymers. It is learned by experience that a spinning solution having good solution state, prepared by use of a good solvent, gives good physical properties to the fiber obtained through coagulation and hot stretching processes. As is clear from the comparison of intrinsic viscosities between dimethylformamide solution and nitric acid solution, it is preferable to use nitric acid of more than 62.5% whose polymer solution shows a comparable intrinsic viscosity to that of dimethylformamide solution. The fiber obtained by coagulating the spinning solution prepared by dissolving the polymer in an organic solvent such as dimethylformamide is usually hot-stretched at higher than about 120 C. From an economical point of view, hot stretching at high temperatures is not profitable. Nevertheless, high temperatures had heretofore been required for producing fibers having satisfactory physical properties. Therefore, to produce the coagulated fiber which can be hot-stretched at around C. is of great importance from an industrial point of view.

In the case where 62.5% nitric acid solution gives the same intrinsic viscosity as that of a dimethylformamide solution and is used as the solvent for making a spinning solution, the coagulated thread obtained therefrom also requires hot-stretching at l30 C. in a super heated steam bath or in glycerol-water systems to obtain satisfactory physical properties or" the fiber.

However, the solution state of dissolved polyacrylonitrile in a spinning solution becomes markedly improved with the increase of nitric acid concentration, and it is quite possible to prepare a coagulated thread which can be hot-stretched at 100 C., by using more than 66% by weight of nitric acid as the solvent. This is an important discovery from an industrial point of view as well as resulting in improved properties of the resulting fiber. Thus, it is an important aspect of this invention to select suitable concentrations of nitric acid to enable coagulated fiber to be hot-stretched at lowertemperatures than herebefore. Moreover, the thus prepared spinning solution must have a viscosity of 700 poises or more at 0 C., while the spinning solution gives markedly favorable re sults, by using a polymer having an average molecular weight of 60,000100,000. However, it is not desirable that the viscosity of the solution exceed 2,000 poises because if the viscosity exceeds 2,000 poises, operations such as the transportation of the solution through a pipe, defoaming or filtration of the solution are extremely difficult. The determination of moleclular weight is most preferably conducted according to the light scattering method, and acrylonitrile polymer of .the desirable molec ular weight containing at least 85% of acrylonitrile shows an intrinsic viscosity of 115 cc./g.-170 cc./g. at 35 C. in dimethylformarnide solution. It is not objectionable that the nitric acid solvent contain some amount of other compounds to such an extent that they do not seriously affect the solution state of the polymer. It should be noted, however, that even when such a high concentrated nitric acid is employed as the solvent, denaturation of the acrylonitrile polymer in the said solution makes it impossible to obtain the benefits of the present invention. In the processes for the production of acrylic fibers by the use of a series of the aforesaid organic solvents, the chemical reaction between the polymer and the solvent is not required to be taken into consideration at all. Therefore, the resulting fibers show chemical properties that are inherent in the polymer. In contrast thereto, when concentrated nitric acid is used as a solvent, the reaction of acrylonitrile polymer and nitric acid should always be taken into consideration. In case nitric acid of more than 66% by weight of HNO is employed, there occurs an unavoidable chemical reaction between the acrylonitrile polymer molecules and the nitric acid. For example, in US. Patent No. 2,928,715 Halbig has recognized that nitration reaction of a polymer occurs when nitric acid of more than 65% HNO is used. It has been considered, however, that the effect of a very Slight nitration reaction on the properties of fiber is considered insignificant as compared with the effects of, for example, hydrolysis of the nitrile group in the polymer chain, cross-linking, and the degradation of polymer. However, the experimental results of the present inventors showed the following facts. The nitration reaction of acrylonitrile polymer in more than 65% nitric acid is promoted by small amounts of nitrous acid (nitrogen dioxide) but it is completely prevented for a definite time required for industrial production, for example, for about 200 hours, in cases where the nitrous acid content in the nitric acid is lowered to less than 0.0005%, and preferably less than 0.0003%. Furthermore, in order to completely prevent the said reaction, the nitrous acid should be removed while also the temperatures of dissolving polymers, of the storing of the resulting spinning solution and of the transferring the spinning solution he maintained at lower than 5 C., and preferably below 3 C. The lower limit of the temperature of the coagulation bath is established as 30 C. because of industrial and economical reasons. Special devices and considerations are necessary for cooling the bath below 30 C. and for spinning the fiber below that temperature. However, within the possible spinning temperature, the lower the temperature of the coagulation bath the better the feel of the yarn produced. As mentioned above, in order to use the nitric acid containing less than 0.0005% nitrous acid as the solvent for acrylonitrile polymers, careful attention should be always paid to the storage of the acid. For example, it may be necessary to keep urea in the nitric acid, from which the nitrous acid has been removed, and keep the acid at a low temperature. Also, a pipe leading to a dissolver must be cooled at a low temperature, too.

Furthermore, the quality of nitric acid itself must be carefully examined before use. For example, nitric acid from which nitrous acid is not removable because of impurities incorporated therein is sometimes found in the market. The inventors experienced many cases where such nitric acid easily reproduced nitrous acid, by the decomposition of nitric acid, even if nitrous acid had been previously removed completely. Practically, the nitric acid, from which nitrous acid can be removed easily, is better adapted to the present invention. For example, if the amount of nitrogen dioxide becomes less than 0.01% when 500 cc. of nitric acid in a fiask was aerated, at 50 C. this nitric acid is preferable for use as a solvent for producing acrylic synthetic fiber. Because of the reasons mentioned hereinbefore, it is very desirable to apply confined conditions to the production of acrylic synthetic fiber with nitric acid solvent. As regards the nitric acid concentration, since the degree of increase of solubility of acrylonitrile polymer in nitric acid decreases when nitric acid concentrations exceed about 70 percent by weight as explained already, no advantage can be expected by using very high concentration of nitric acid which thereby avoids the economical disadvantage brought about by the use of such a high concentration. Moreover, if the nitric acid concentration exceeds percent there arises, although slightly, several undesirable reactions between the molecular chain of polyacrylonitrile and nitric acid which have not yet been solved. These reactions, although slight, spoil the good feel of the acrylonitrile fiber. Because of the above mentioned reasons, the upper limit of the concentration of nitric acid solvent is established as 80 percent. As the second necessary condition for obtaining the benefits of the present invention, the spinning solution prepared under the above conditions should be spun by extruding into a coagulation bath containing 28% to 36% aqueous nitric acid solution kept at or below 5 C. The coagulation conditions as well as the solution state of the polymer in a spinning solution are important factors which affect the fine gel structure of the coagulated thread. For example, when the temperature of the coagulation bath is kept at above 5 C. or when the concentration is less than 28% or more than 36%, the resultant product does not have any of the desirable characteristics of the thread according to the invention. The coagulation thread thus obtained is further passed through a dilute aqueous nitric acid solution of 1520% (by weight) and then through water to wash the acid. It is not always necessary to keep the second coagulation bath below 5 C., but it is desirable not to raise the temperature above room temperature. Furthermore, for attaining the favorable benefits of the present invention, a shrinkage of 15% to 50% should be given to the thread in the first coagulation bath during the process of coagulation. In the second coagulation bath, however, no such shrinkage is particularly required.

The thus coagulated thread, after washing with water to remove acid therefrom, is subjected to hot stretching in hot water at C. or below so as to stretch the thread at least 7 times. Also, it is possible to practice hot stretching by using multi-stage stretch baths and raising the bath temperatures in successive order. But, when the total wet hot stretch of the thread exceeds 11, the benefits of the present invention will be obstructed. A temperature which enables stretching the thread at least 7 times the original length of the coagulated thread in a wet process is sufiicient as a stretch temperature. For this purpose 80 C. or higher is generally suitable. HOW- ever when hot stretching is practiced in multi-stages the temperature of the first bath can be about 60 C. In this case, a small amount of inorganic salt, organic compound, inorganic or organic acid may be added to the hot-stretch medium, which generally is water, in such an amount that it does not harm the hot stretching characteristics. However, the employment of a concentrated inorganic salt solution bath is undesirable. After heat stretching, the resulting wet fiber should necessarily be dried with air heated at least to above 80 C. to obtain the benefits of the invention. The fiber dried at a temperature of less than 80 C. would have no benefits of the invention. If normal pressure is employed for the drying, the temperature applied to the fiber does not exceed 100 C. and therefore a temperature of hot air below C. is sutficient. Proper heat treatment of the fiber after drying does not impair the benefits of the present invention.

The above mentioned various conditions for spinningindicate combined and related conditions for producing a fiber having good feel. The factors which affect this feel in the spinning solution were considered in connection with the dissolved state of acrylonitrile polymer, but in spinning these factors may be explained in connection with the micro structure of the coagulated thread or gelled filamental material. For example the fact that, the viscosity of the spinning solution must exceed 700 poises seem to affect the condition of the micro structure. Also the conditions of the hot stretching have close relations to the deformation of this micro structure. Although the expression, feeling is not proper to indicate fixed values, it is used as a measure to determine the above mentioned conditions for the production of acrylonitrile fiber. Mechanical properties of a fiber such as tensile strength, elasticity and brittleness are not suitable to express the feel of a fiber. Accordingly, it is diificult to compare numerically the feel of a fiber produced within the conditions of the present invention and a fiber produced under other conditions.

In carrying out the present invention, acrylonitrile polymer or its copolymer is prepared by a known method. In case of preparing a copolymer of acrylonitrile, a mixture of monomers composed mainly of acrylonitrile is used. It is preferable that the amount of acrylonitrile be at least about 85 and the amount of copolymerizable monomer be less than 15% by weight of the total monomer mixture. If the amount of acrylonitrile in the monomer mixture is less than about 85% by weight, the benefits of the present invention will not sufficiently be obtained even if the copolymer is treated in accordance with the present invention. Typical examples of monomers copolymerizable with acrylonitrile to produce copolymers are compounds having simple CH =CH radicals. The following are offered by way of example: Vinyl esters, especially vinyl esters of saturated aliphatic mono-carboxylic acid such as vinyl acetate, vinyl propionate, vinyl butyrate and the like; acrylic acid and substituted acrylic acid (for example, methacrylic acid and ethacrylic acid); esters and amides of these acids (for example, methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, methyl methacrylate, ethyl methacrylate, propyl methacrylate, butyl methacrylate, acryl amides, methacryl amides, N-methyl acrylamide, N-ethyl acrylamide, N-pr-opyl acrylamide, N-butyl acrylamide, N-methyl methacrylamide, N-ethyl methacrylamide, N-propyl methacrylamide, and N-butyl, methacrylamide and the like); methacrylonitrile, ethacrylonitrile and other hydrocarbonsubstituted acrylonitriles, and other various vinyl and acryl compounds which may give a thermoplastic copolymer by the copolymerization with acrylonitrile. Alkyl esters of alpha or beta unsaturated polycarboxylic acid, and vinyl compounds containing sulfonic acid radicals may also be employed for the copolymerization with acrylonitrile to produce copolymers useful for the practice of the present invention.

For the production of the polymer and copolymer in the present invention, the usual catalysts are used and these may be ammonium or alkali persulphate, perboric acid and percarbonic acid, hydrogen peroxide, benzoyl peroxide, substituted benzoyl peroxide with one or more substituents in the benzoyl nucleus, lauroyl peroxide, lauroyl benzoyl peroxide, t-butyl peroxide, cumene hydroperoxide, and azo compounds such as 2,2'-azo-bis-isobutyronitrile. As polymerization assistants, mercaptans such as lauryl mercaptan, d-mercaptoethanol, thioglycolic acid and the like, and other proper dispersing agents may be employed. For the polymerization reaction, solvents such as water and other proper organic solvents may also be employed. Of course, the benefits attained by the method of this invention are not only limited to polymers prepared by said polymerization method, but it is also applicable to polymers obtained'by other polymerization- Example 1 A spinning solution was prepared by dissolving acrylonitrile polymer composed of acrylonitrile 5% methyl acrylate and 5% vinyl acetate having an average molecular weight of 85,000, into 70% nitric acid. The solvent of 70% nitric acid, was purified beforehand until the nitrous acid content was lowered to 0.004% by the addition of hydrogen peroxide and urea. The polymer was dissolved in the said nitric acid at a temperature of 3 C. and the temperature of .the steps for transferring the spinning solution to the spinning machine, defoaming, and others were kept at between 5 and 1 C. The polymer was dissolved in the solvent to give a viscosity of 1100 poises at 0 C. This spinning solution was extruded through a 2000 hole spinneret wih holes 0.1 mm. in diameter, into a 30% aqueous solution of acid at 3 C., at a linear velocity of 7 m. per minute and was coagulated. During the coagulation, the fiber was sub jected to shrinkage of 18%. Then the fiber was transferred into a second coagulation bath comprising a 20% aqueous nitric acid solution at 15 C. and was then washed with water. In the second coagulation bath as well as in the water bath, the rollers advancing the fibers were adjusted so that no shrinkage of the fiber occurs. The thus obtained coagulated fiber was next subjected to heat stretching in hot water at 90 C. (the pH of the water was adjusted to 3 with phosphoric acid) to stretch the fiber 9 times its length, and the resulting stretched fiber was dried in hot air at 130 C. The fiber thus obtained had an extremely soft touch. However, the feel of the fiber produced under the same conditions as mentioned above except where 65% nitric acid was used as the solvent in the spinning solution was not as good as compared with that of above-mentioned fiber.

Example 2 A binary copolymer composed of 93% of acrylonitrile and 7% of methyl acrylate with an average molecular weight of 77,000 was dissolved into the same nitric acid as that of Example 1 at 0 C. to prepare a spinning solution with a viscosity of 800 poises at 0 C. The spinning solution was kept at a temperature between 0 C. and 5 C. until it reached the spinneret, and was then extruded through the spinneret into a 32% aqueous nitric acid solution, while being subjected to a shrinkage of 15%. The fiber was then led into a 15% aqueous nitric acid solution, washed with water, heat stretched to 8 times its length in hot water at C., and then dried in hot air at 100 C. The obtained fiber had an extremely good feel. However, the feel of the fiber, obtained under the same conditions as above except where 70% nitric acid containing 0.03% of nitrous acid was employed as a solvent, was not as good as compared with that of the above-said fiber.

Example 3 A ternary copolymer composed of 92% acrylonitrile, 7% methyl acrylate, and 1% of sodium methallyl sulfonate with an average molecular weight of 90,000, was dissolved in a 68% nitric acid solvent at 0 C. to prepare a spinning solution. This solution had a viscosity of 1,300 poises at 0 C. The nitric acid was thoroughly purified beforehand until the nitrous acid content was lowered to 0.0002%. After dissolving, all the steps were conducted at 2 C. The spinning solution was extruded through a spinneret with holes of 0.08 mm. in the diameter into 33% nitric acid at -5 C. to coagulate it while giving a shrinkage of 20%. Then, the fiber was introduced into 19% nitric acid at 15 C., washed with water to remove as much of the acid as possible. The fiber was then heat stretched 4 times its original length in hot water at 70 C., and then 2 times in hot water at 100 C. The fiber was next dried in hot air at 150 C. The resulting fiber had an extremely good feel, but a fiber prepared by heat-stretching only in the first stretch-bath by 4 times or a fiber prepared with an additional stretching of 3 times in the second step, had not as good a feel as compared with that of the above-mentioned fiber.

What we claim is:

In a process for producing synthetic fibers by wetspinning an acrylonitrile polymer having an average molecular Weight between 60,000 and 100,000 and containing at least 85% by weight of acrylonitrile with a balance of ethylenically unsaturated hydrocarbon comonomer according to a nitric acid method, an improvement which comprises: dissolving the acrylonitrilic polymer at a temperature of from 30 C. to 5 C. in a solvent containing nitric acid of more than 66% and less than 80% by weight from which nitrous acid has been removed so that the content thereof is less than 0.0005 to prepare a spinning solution with a viscosity of between 700 and 2000 poises at 0 C., extruding said spinning solution into an aqueous coagulation bath kept at between C. and 5 C. and containing between 28 and 36% nitric acid to coagulate the solution to form filaments while allowing shrinkage of at least 15% and not more than 40% to the resulting filaments, treating the filaments with an aqueous solution of 15 to 25% nitric acid by Weight, Washing With Water, stretching the filaments in hot water at a temperature less than 100 C. and not less than C. to stretch the filaments between 7 to 11 times; in length, and then drying the filaments at a temperature of -100 C.

References Cited in the file of this patent UNITED STATES PATENTS 2,579,451 Poison Dec. 18, 1951 2,878,097 Halbig Mar. 17, 1959 2,928,715 Halbig Mar. 15, 1960 FOREIGN PATENTS 744,278 Great Britain Feb. 1, 1956 

